Turntable sibilance

"but how do you change the vta on these arms"

There's no adjustment for it. You have to add spacers if you want to change it. It may be a good idea to list your system.

The Michell VTA adjuster available from musicdirect.com works pretty well if your arm doesn't have and adjuster.

http://www.musicdirect.com/p-7336-michell-rega-vta-adjuster-for-michell-turntables.aspx

It should make a difference you can hear. Also have you checked the cartridge alignment with an alignment gauge?

I think the green slider you mentioned is the anti-skate adjustment which should be set to the cartridge tracking weight/force or slightly lower.

Beyond VTA, there are two things that would manifest as sibilance (not having heard the actual rig of course):

Mistracking from either VTF or anti-skate set incorrectly. Since you've tried VTF, then it could be anti-skate (although I doubt it). Try playing individually through each speaker--if you notice a tonal difference (or distortion) through one channel vs. the other, that suggests anti-skate could be an issue.

Phono stage loading--if the loading is not matched to the cartridge, the tonal balance will be affected. Either muddy and closed in, or bright, harsh and sibilant. I think this cart should be loaded between 100 and 150 ohms.

Good luck--I'm sure you'll ultimate get the right answers here in the forum!

Kj...vtf maybe...antiskate...no way.

With all due respect, the first two responders must not be familiar with the Nottingham Horizon table. While they are correct that the Rega arm itself does not have adjustable VTA, the arm mount of the Horizon provides that adjustment. There are two small hex head bolts on the side of the round silver colored collet that the arm slides into. When you loosen those you can then raise or lower the tonearm into the desired position and then snug them up to set the VTA. Do not overtighten them, just make them firm enough to hold their position. To reduce high frequency response and sibilance would usually require the arm height to be lowered. Then also recheck alignment and tracking force to ensure they're still set correctly.